Method of making a multialkali electron emissive layer

ABSTRACT

An S-20 response multialkali photocathode is produced by exposing a layer of antimony to activating vapors of potassium, sodium, and cesium. In one method the antimony layer is exposed to cesium at a temperature of about 240*C. prior to exposure to potassium or sodium, and the exposing to cesium is terminated before the photosensitivity reaches a peak. In another method, the antimony layer is exposed to cesium at a temperature of about 240*C. subsequent to exposure to potassium and sodium, and the exposing to cesium is terminated when the photosensitivity reaches a peak.

United States Patent [191 Summer 1 May 20, 1975 METHOD OF MAKING A MULTIALKALI ELECTRON EMISSIVE LAYER [75] Inventor: Alfred Hermann Summer,

Princeton, NJ.

[73] Assignee: RCA Corporation, New York, NY.

[22] Filed: Mar. 4, 1974 [2]] Appl. No.: 447,936

Related US. Application Data [63] Continuation-inpart of Ser. No. 314,001, Dec. 11,

1972, abandoned.

[52] US. Cl. 316/6; 117/215; 117/219;

316/3; 316/5 [51] Int. Cl. HOlj 9/38 [58] Field of Search 316/3, 5, 6, 8, 10, 12,

[56] References Cited UNITED STATES PATENTS 2,045,418 6/1936 Simms 316/6 2,192,418 3/1940 Sommer 316/1 X 2,206,372 7/1940 Sommer 316/12 2,401,734 6/1946 Janes 316/5 X 2,676,282 4/1954 Polkosky 117/211 X 2,779,888 1/1957 Stoudenheimer..... 313/103 2,881,343 4/1959 Polkosky 117/224 3,023,131 2/1962 Cassman 117/211 X 3,434,876 3/1969 Stoudenheimer 117/219 X 3,498,834 3/1970 Rome et a1v 117/225 X 3,658,400 4/1972 Helvy 316/5 X Primary Examiner-Roy Lake Assistant Examiner-James W. Davie Attorney, Agent, or Firm-G. H. Bruestle; R. J. Boivin [57] ABSTRACT 11 Claims, 2 Drawing Figures METHOD OF MAKING A MULTIALKALI ELECTRON EMISSIVE LAYER The application is a continuation-in-part of application Ser. No. 3l4.00l. filed Dec. ll, 1972. now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to the processing of electron-emissive, or cathode layers comprising a layer of antimony activated with several alkali metals.

Electron-emissive layers are used, for example, as photocathodes or secondary electron-emitting layers in phototubes, image tubes, camera tubes and various other devices. One type of photocathode comprises antimony combined with potassium, sodium, and cesium. Such a photocathode is generally referred to as a multialkali or an S20" photocathode and has a characteristic spectral response known as an S-20 response. Relatively high efficiency 5-20 photocathodes are made by a rather complex activation process including a number of evaporations of antimony on a predeposited layer of antimony during activation and exposure of the cathode at elevated temperatures to potassium, sodium. and cesium vapor between the evaporations. The exposures to potassium and to sodium are at temperatures on the order of 200C. (Celsius), while the exposures to cesium are at temperatures on the order of l60C. Such processes are described, for instance in US Pat. No. 3,658,400 issued to Helvy on Apr. 25, I972.

In some tubes, however, while it is desirable to have an S photocathode, the configuration of the tube interior does not permit evaporation of a uniform layer of antimony inside the tube during activation of the cathode. For example, in certain photomultipliers and proximity focussed image tubes, an antimony evaporation source cannot be located sufficiently distant from the photocathode to deposit a uniform layer of antimony inside the tube during processing, either because the tube itself is not large enough. or because an electrode blocks the source. In these tubes. all the antimony must be pre-deposited on the photocathode substrate prior to tube assembly. Activation of the predeposited, antimony layer according to prior art practices by exposure to potassium, sodium, and cesium.

and at the elevated temperatures generally used in activation of multiple antimony layer photocathodes yields a photocathode of unacceptably low sensitivity.

SUMMARY OF THE INVENTION In thenovel process an antimony layer is activated with a plurality of alkali metals including cesium wherein exposure of the antimony layer to activating vapor of cesium is at a temperature exceeding about 200C. Specifically, the activation of an antimony layer comprises exposing the layer to cesium vapor at a temperature exceeding 200C. either prior to or subsequent to exposing it to potassium and sodium vapors. If the exposure to cesium is accomplished before exposing the layer to potassium and sodium. the exposure to cesium is terminated before the photosensitivit reaches a peak.

With the novel process, the photosensitivity of the single antimony layer type of multialkali photocathode is greatly increased. thus making it commercially feasible to use S-20 response multialkali photocathodes in devices in which the geometry does not permit evaporations of antimony during activation processing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a break-away, side view ofa photomultiplier utilizing a photocathode made by the novel process.

FIG. 2 is a sectional view of the tube of FIG. 1 taken at line 22 of FIG. I.

PREFERRED EMBODIM ENT In a preferred embodiment of the invention, the photomultiplier tube 10, shown in FIGS. 1 and 2, is processed by the novel method.

Referring now to FIG. 1, the tube 10 has a glass envelope 12 with a stem portion 14. The stem portion I4 includes a length of exhaust tubulation l6 and a number of embedded metal pins 18 for making electrical contact to the interior of the tube 10.

Mounted inside the tube 10 is an electrode assembly shown in more detail in FIG. 2. The assembly includes a metal photocathode substrate 20 having an evaporated antimony photocathode layer 22 about 10- l00nm (nanometers) thick on one face. Angularly disposed from the substrate is a mesh electrode 24, which is followed, proceeding counterclockwise about the tube axis, by a series of 8 dynodes 26 and an anode 28 with an anode shield 30. The general direction of travel of electrons emitted from the photocathode 22 is indicated by the dashed directional lines 32. Between the electrode assembly and the stem 14 are a potassium channel 34, a sodium channel 36 and a cesium channel 38 which can be electrically-resistance-heated for releasing their respective alkali metal vapors into the interior of the tube 10. I 7

As a preliminary step to activation of the photocathode layer 22, an oven is placed around the tube 10 and the tube 10 is baked-out for several hours at a temperature of about 300C. to eliminate contaminants from the interior. During the entire processing, the interior of the tube 10 is continuously evacuated through the exhaust tubulation 16. After the tube 10 is baked out, the activation of the photocathode layer 22 may proceed as follows, with the photosensitivity of the photocathode 22 being monitored during prQcessin g generally as described, for example in US Pat. No. 2,676,282 issued Apr. 20, I954 to J. J. Polkosky EXAMPLE 1 l. The temperature of the tube 10 is lowered from the bake-out temperature to about 240C., and the cesium channel 38 heated so that a small amount of cesium vapor is released into the interior of the tube 10 until the photosensitivity reaches a value of approxi mately 0.1 microampere. per lumen. Vaporization heating of the cesium channel is then terminated.

2. Next, the tube 10 is maintained at about 240C. and the potassium channel 34 heated to release potassium vapor into the tube 10 until the photosensitivity reaches a peak value, generally somewhat below 10 microamperes per lumen. vaporization heating of the potassium channel is then terminated.

3. With the tube 10 still maintained at about 240C, thesodium channel 36 is now heated to release sodium vapor until another peak photosensitivity is reached. The vaporization heating of the sodium channel is then terminated, and the tube 10 is cooled to room temperature and the exhaust tubulation l6 sealed off.

EXAMPLE 2 l. The temperature of the tube is lowered from the bake-out temperature to a temperature which exceed's about 160C. as, for example, about 240C. and the potassium channel heated to release potassium vapor into the tube 10 until the photosensitivity reaches a peak value. Vaporization of the potassium channel is then terminated.

2. Next, the tube is maintained or heated to a temperature above 220C. as, for example, about 240C. and the sodium channel 36 is heated to release sodium vapor until another peak photosensitivity is reached. The vaporization heating of the sodium channel is then terminated.

3. With the tube 10 heated to or maintained at a temperature about 240C. the cesium channel 38 is heated so that cesium vapor is released into the interior of the tube 10 until photosensitivity reaches a peak value. The vaporization heating of the cesium channel is terminated and the tube 10 is cooled to room temperature, and the exhaust tubulation l6 sealed off.

GENERAL CONSIDERATIONS 1n the prior art methods of activating antimony with sodium, potassium, and cesium, the cathode is maintained at a temperature on the order of only about 160C. during activation with cesium vapor and the exposure is continued until the photosensitivity of the cathode is at or past a peak.

in the novel process, on the other hand, it is critical that the temperature of the cathode layer during the ex-' posure to the cesium vapor be above about 200C., which is higher than temperatures generally specified for the cesium in prior activation methods. Also, with the novel process, during exposure to each of cesium, potassium; and sodium a uniform temperature may be maintained without modification at between about 220C. and about 240C.

Regarding Example 1, it is important that the amount of cesium generated not be excessive. It is preferred that the cesium be generated until the photosensitivity of the cathode reaches a value on the order of only l/lO microampere per lumen, rather than to peak sensitivity, which would occur at a value of about microamperes per lumen. While this particular value of 1/10 microampere per lumen may not be highly critical, it has been found, for instance, that if cesium is released until the photosensitivity is higher than about one-half microampere per lumen, the resulting photocathode is substantially inferior to one in which the cesium is released only until a value of about l/lO microampere per lumen.

The method of Example 1 has been found to produce electron emissive layers, generally superior to those obtained by prior art methods of activating an antimony layer. in sequence, with the vapors of cesium, potassium vapors; wherein, a cesiu m activation is accomplished at very high temperature exceeding about 200C. and preferably about 240C.

While the process has been described for'making a photocathode, the product of the process is an elec tron-emissive layer generally, whether the emission is primary or secondary.

1 claim:

1. A method of activating a multialkali electronemissive cathode layer, of the type wherein a layer of antimony is exposed at elevated temperature, within an evacuated body, to vapors of a plurality of alkali metals including cesium, to form an e1ectronemissive compound, wherein the improvement comprises:

exposing the cathode layer to cesium vapor to achieve a photosensitivity exceeding approximately 0.1 microampere per lumen while maintaining the cathode at a temperature above 200C.

2. The method defined in claim 1, wherein the antimony layer is activated by vapors of sodium, potassium, and cesium.

3. The method defined in claim 2, additionally comprising the steps of:

exposing the cathode layer to cesium vapor prior to exposing it to potassium and sodium vapors, and terminating said exposing to cesium before the photosensitivity of the cathode reaches a peak. 4. The method defined in claim 3, wherein said terminating is before the photosensitivity of the cathode reaches about one-half microampere per lumen.

5. The method defined in claim 4, wherein the cathode is at a temperature above 200C. during the exposing to sodium and potassium vapors.

6. The method defined in claim 2, comprising: exposing the cathode layer to cesium vapor until the photosensitivity of the exposed cathode layer is about one-tenth microampere per lumen, while maintaining the temperature of the cathode layer at between 220C. and about 240C.; then exposing the cathode layer to potassium vapor until the photosensitivity of the exposed cathode layer reaches a peak, while maintaining the temperature of the cathode layer at between about 220C. and about 240C.; then exposing the cathode layer to sodium vapor until the photosensitivity of the exposed cathode layer reaches a peak, while maintaining the temperature of the cathode layer at between about 220C. and about 240C.

7. The method defined in claim 2, additionally comprising the step of:

exposing the cathode layer, in sequence, with potassium, sodium, and cesium vapors.

8. The method defined in claim 7, additionally comprising the step of:

terminating said exposing to each of the vapors of potassium, sodium, and cesium vapor when the photosensitivity of the cathode reaches a peak.

9. The method defined in claim 8, wherein said cathode is maintained at a temperature of between about 220C. and about 240C. during said activation of said cathode with each of the vapors of sodium, and cesium.

10. The method defined in claim 9, wherein said cathode is maintained at a temperature exceeding about C. during said activation with potassium vapor.

11. The method defined in claim 2, comprising:

6 of the cathode layer within the range of about 220C. and about 240C. exposing the cathode, next, to cesium vapor until the photosensitivity of the exposed cathode reaches a peak. while maintaining the temperature of the cathode layer within the range of about 200C. and about 240C. 

1. A METHOD OF ACTIVATING A MULTIALKALI ELECTRON-EMISSIVE CATHODE LAYER, OF THE TYPE WHEREIN A LAYER OF ANTIMONY IS EXPOSED AT ELEVATED TEMPERATURE, WITHIN AN EVACUATED BODY, TO VAPORS OF A PLURALITY OF ALKALI METALS INCLUDING CESIUM, TO FORM AN ELECTRON-EMISSIVE COMPOUND, WHEREIN THE IMPROVEMENT COMPRISES: EXPOSING THE CATHODE LAYER TO CESIUM VAPOR TO ACHIEVE A PHOTOSENSITIVITY EXCEEDING APPROXIMATELY 0.1 MICROAMPERE PER LUMEN WHILE MAINTAINING THE CATHODE AT A TEMPERATURE ABOVE 200*C.
 2. The method defined in claim 1, wherein the antimony layer is activated by vapors of sodium, potassium, and cesium.
 3. The method defined in claim 2, additionally comprising the steps of: exposing the cathode layer to cesium vapor prior to exposing it to potassium and sodium vapors, and terminating said exposing to cesium before the photosensitivity of the cathode reaches a peak.
 4. The method defined in claim 3, wherein said terminating is before the photosensitivity of the cathode reaches about one-half microampere per lumen.
 5. The method defined in claim 4, wherein the cathode is at a temperature above 200*C. during the exposing to sodium and potassium vapors.
 6. The method defined in claim 2, comprising: exposing the cathode layer to cesium vapor until the photosensitivity of the exposed cathode layer is about one-tenth microampere per lumen, while maintaining the temperature of the cathode layer at between 220*C. and about 240*C.; then exposing the cathode layer to potassium vapor until the photosensitivity of the exposed cathode layer reaches a peak, while maintaining the temperature of the cathode layer at between about 220*C. and about 240*C.; then exposing the cathode layer to sodium vapor until the photosensitivity of the exposed cathode layer reaches a peak, while maintaining the temperature of the cathode layer at between about 220*C. and about 240*C.
 7. The method defined in claim 2, additionally comprising the step of: exposing the cathode layer, in sequence, with potassium, sodium, and cesium vapors.
 8. The method defined in claim 7, additionally comprising the step of: terminating said exposing to each of the vapors of potassium, sodium, and cesium vapor when the photosensitivity of the cathode reaches a peak.
 9. The method defined in claim 8, wherein said cathode is maintained at a temperature of between about 220*C. and about 240*C. during said activation of said cathode with each of the vapors of sodium, and cesium.
 10. The method defined in claim 9, wherein said cathode is maintained at a temperature exceeding about 160*C. during said activation with potassium vapor.
 11. The method defined in claim 2, comprising: exposing the cathode layer to potassium vapor until the photosensitivity of the exposed cathode layer reaches a peak, while maintaining the temperature of the cathode layer within the range of about 160*C. and about 240*C. exposing the cathode layer, next, to sodium vapor until the photosensitivity of the exposed cathode reaches a peak, while maintaining the temperature of the cathode layer within the range of about 220*C. and about 240*C. exposing the cathode, next, to cesium vapor until the photosensitivity of the exposed cathode reaches a peak, while maintaining the temperature of the cathode layer within the range of about 200*C. and about 240*C. 